1. Field of the Invention
The present invention relates to a developing apparatus, a developing method, an image-forming apparatus, and an image-forming method each utilizing electrophotography.
2. Description of the Related Art
Although many methods have each been known as electrophotography, a general method is as described below. An electrostatic latent image is formed on an electrostatic latent image bearing member utilizing a photoconductive material by various units. Next, the electrostatic latent image is developed with toner to form a toner image and the toner image is transferred onto a recording medium such as paper. After that, the toner image is fixed onto the recording medium by heat and/or a pressure to provide a copy. A copying machine, a printer, or the like is available as an image-forming apparatus utilizing such electrophotography.
The transition of such printer or copying machine from an analog system to a digital system has progressed in recent years, and hence the printer or the copying machine has been required to have excellent electrostatic latent image reproducibility and a high resolution. In addition, the downsizing of, in particular, the printer has been strongly required.
The printer has heretofore been used in the following manner in many cases. The printer is connected to the network and many persons perform printing with the printer. However, a demand for the following manner has been growing in recent years. A PC and the printer are placed on an individual's desk, and the individual performs printing at hand. To this end, the space savings of the printer is necessary and hence the requirement for the downsizing of the printer has been strong.
In addition, even such compact printer has been strongly desired to have high image quality and a small variation in image quality (high stability) not only under a normal-temperature and normal-humidity environment but also under a high-temperature and high-humidity environment or the like.
Here, attention is paid to the downsizing of the printer. The downsizing of a fixing apparatus and the downsizing of a developing apparatus are mainly effective for the downsizing. In particular, the latter accounts for a considerable portion of the volume of the printer and hence the downsizing of the developing apparatus can be said to be essential for the downsizing of the printer.
A development system is considered. A two-component development system or a one-component development system is available as the development system of the printer, but the one-component development system is suitable in a sense that a compact printer can be achieved. This is because the one-component development system is a development system using no carrier.
Next, the downsizing of a developing apparatus adopting the one-component development system is considered. A reduction in diameter of an electrostatic latent image bearing member or a toner carrier is effective for the downsizing of the developing apparatus. In addition, a development system in which the toner carrier and the electrostatic latent image bearing member are placed so as to be brought into contact with each other (placed so as to abut with each other) (hereinafter sometimes referred to as “contact development system”) is preferred from the viewpoint of high image quality.
The following attempt has been made as additional downsizing of a developing apparatus adopting such contact development system (see Japanese Patent Application Laid-Open No. 2005-173484 and Japanese Patent Application Laid-Open No. 2006-154093). The apparatus is downsized by avoiding the use of a toner-supplying member to be placed so as to be brought into contact with the toner carrier.
However, in such developing apparatus, problems peculiar thereto are liable to become tangible.
One of the problems is a problem called reversal fogging. Fogging is a problem in which a toner is present in a non-image portion as a region where the toner is not intended to be developed and hence the non-image portion is contaminated.
Of such phenomena called fogging, the reversal fogging is specifically the following case. For example, when the charging of the toner on the toner carrier is insufficient or when the toner is reversely charged by a certain reason (e.g., when a negatively chargeable toner is positively charged), the toner shifts to the non-image region of the electrostatic latent image bearing member and is transferred onto a recording medium such as paper.
In particular, in a downsized developing apparatus, the curvature of its toner carrier increases in association with the downsizing of the toner carrier. Accordingly, the area of a regulating portion where the toner carrier and a toner-regulating member (hereinafter sometimes simply referred to as “regulating member”) abut with each other reduces, and hence it becomes difficult for a toner to undergo triboelectric charging.
When a toner-supplying member is not used in addition to the downsizing of the developing apparatus, there is no opportunity for the triboelectric charging to occur between the toner-supplying member and the toner carrier, and hence it becomes additionally difficult for the toner to undergo the triboelectric charging.
Further, such reversal fogging is particularly remarkable under a high-temperature and high-humidity environment, and when a difference between a charging bias to be applied to the electrostatic latent image bearing member and a developing bias to be applied to the toner carrier is large.
This is because of, for example, the following reasons. Under the high-temperature and high-humidity environment, the toner is hardly charged, and an insufficiently charged toner is liable to be shifted to the non-image region of the electrostatic latent image bearing member by an electric field resulting from the fact that the difference between the charging bias and the developing bias is large.
A magnetic toner whose dielectric loss factor (∈″) and dielectric tangent (tan δ) have been specified has been proposed as an attempt to improve the chargeability of the toner under the high-temperature and high-humidity environment (see Japanese Patent Application Laid-Open No. 2012-014166).
However, its effect is still insufficient and hence the toner has been susceptible to improvement.